Power generation systems often include a power converter that is configured to convert an input power into a suitable power for application to a load, such as a generator, motor, electrical grid, or other suitable load. For instance, a power generation system, such as a wind turbine system, may include a power converter for converting variable frequency alternating current power generated at the generator into alternating current power at a grid frequency (e.g. 50 Hz or 60 Hz) for application to a utility grid. An exemplary power generation system may generate AC power using a wind-driven doubly fed induction generator (DFIG). A power converter can regulate the flow of electrical power between the DFIG and the grid.
In many instances, power generation systems may be located in remote areas far from the loads they serve. This is particularly true for renewable energy sources, such as wind turbine systems, solar/photovoltaic systems, hydroelectric systems and/or the like. Typically, such power generation systems are connected to the electrical grid through an electrical system including long transmission lines connected to the grid using one or more breakers. As a result, from the standpoint of the power generation system, the electrical grid is relatively weak due to the substantially high impedance of the electrical system.
During operation of a wind turbine system, the generator has a tendency to speed up during the occurrence of a transient event (e.g., a wind gust). As the generator speed increases, the output current of the wind turbine must also be increased in order to maintain a constant torque. In addition, as the output current increases, the converter controller typically attempts to maintain the resulting output power factor of the wind turbine system within the limits defined by the power factor operating requirement associated with the system. For instance, a wind turbine system may be required to operate such that it maintains an output power factor at or near unity power factor, such as a power factor ranging between 0.9 and 1.0. For a wind turbine system connected to a weak grid, as the wind turbine produces current at a power factor that is determined by the converter controller to be at or near unity power factor, the operating constraints resulting from the power factor operating requirement may actually cause in a reduction of the real power delivered to the grid. This results from the fact that the high grid impedance of the weak grid causes the local grid or reference voltage for the system to drop as the current is applied at or near unity power factor, at least from the reference frame of the converter controller. Such a condition may, in turn, lead to a runway overspeed of the generator, which can damage the generator and/or other components of the system.
Accordingly, a system and method for controlling a power generation system connected to a weak grid when such system is subject to a power factor operating requirement that results in reduced power output at increased generator speeds would be welcomed in the technology.